Method of making banded pressure vessels



Feb. 12, 1952 c. H. GAY

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METHOD OF MAKING BANDED PRESSURE VESSELS Filed April 3, 1946 8 Sheets-Sheet 2 INVENTOR.

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METHOD OF MAKING BANDED PRESSURE VESSELS 8 Sheets-Sheet 8 Filed April 3, 1946 Patented Feb. l2, 1952 STATES PATENT OFFICE METHOD F MAKING BAND-ED PRESSURE VESSELS Cecil H. Gay, Akron, Ohio, asslgnor to The Babcock & Wilcox Company, Bockleigh, N. J., a corporation of New Jersey Application April s', 194s, serial No. 659,269 i claim. (ci. 2er-148.2)

My invention relates to methods of fabricating banded pressure vessels. f

The invention is particularly concerned with a method of fabricating pressure vessels which are adapted for advantageous use in industries in- 5 welds, when not X-rayed and annealed, with unit VOlving- OperatOns at high fluid pressures- The design stresses in the metal of the weld, limited walls of pressure vessels for withstanding such to 80% of the metal of the adjacent sheet or plate?l pressures must be of such great thickness that As the stresses in the metal of the wall of a many diiculties have been encountered in their vessel subjected to internal pressure are greatest fabrication. For example, it has been proposed at the internal wall and decrease progressively to forge such pressure vessels from a single ingot, to the outer layers or laminae, it will be evident but this process has been found to involve almost that outer layers are subject to much lower unit prohibitive expense. One reason for this is that stresses, and by the selection of the thickness the forging process involves numerous beatings of the inner welded, X-rayed and heat treated and reheatings of the metal in order to maintain cylinder with respect to the thickness of the it at forging temperature throughout the manuadded bands, it is possible according to the pres-n facturing operations. The pertinent pressure ent invention to attain a maximum unit stress vessel eliminates such operations. in the bands which is 80% of the permissible unit The invention presents a method of manufacstress for the metal used in the body portion of turing a composite pressure vessel construction the weld. the inner part of which includes a welded shell The resulting pressure vessel is one in which of 'uniform plate thickness with hemispherical the average stress throughout the wall thickness heads of the same plate thickness at their posiof its cylindrical section is approximately the tions of attachment to the cylindrical shell, the same as that required for a single Wall thickness plate thickness of the shell being determined by vessel, welded, stress relieved, and X-rayed, althe thickness of the plate in the heads. The though more than 50% of the wall thickness reshell is fabricated by forming plate sections to quires no stress relieving or X-raying- Furthershell diameter and then welding them into annumore, the total wall thickness of the cylindrical lar shell sections. The latter are united by girth section of the illustrative pressure vessel is no welds at their meeting edges and the hemispherigreater than that required fOr a Unitary plate cal heads are then welded thereto, The welds welded construction which has been totally stress are X-rayed and if any defects are found the relieved and X-rayed. defective welds are removed and replaced. The Other objects of the invention will appear in construction is then stress relieved. For pressure the following description which refers to the acvessels thus fabricated such standards as those Companying drawings. of the A. S. M. E. code for unred pressure vessels In the drawings: limit the permissible value of the design stress Fig. l is a longitudinal vertical section of the on the welds to not more than 90% of that allowillustrative pressure vessel; able for the plate metal of the components of the Fig. 2 is a transverse vertical section on the vessel joined by the welds. line 2 2 of Fig. 1

The remainder of the pertinent pressure vessel Fig. 3 is a detail view showing the oiset arconsists of successively applied and substantially rangement of the longitudinal welds in successive concentric bands, of sheet metal or metallic plates laminae of the outer part of the wall of the cyeach individually clamped circumferentially upon lindrical section of the illustrative pressure vesthe shell and secured by a longitudinal seam weld sel; formed between its ends while the band is held Fig. 4 is a detail view showing a section of a in clamped condition. Preferably, each band is girth weld in the inner shell construction and inheated before the welding and the tightening of 4dicating the lack of similar welds between the the band is such that the shell is placed under bands of successive sections of the outer shell; compressive stress. These bands are applied 5o Fig. 5 is a partial vertical section (longitudithroughout the length of the shell, and a pluralnally of the pressure vessel) showing the modiity of them are directly superimposed and arcation of the construction of a section of the ranged to form separated sections of superlmillustrative pressure vessel associated with a side posed bands along the length of the shell. This nozzle welded to the inner shell and extending results in a multiple layer construction without through additional bands;

girth welds between the bands of successive sections.

Such standards as the A. S. M. E. code for pressure vessels permits the design of pressure vessel pressure vessel wall; y

Fig. 9 is a diagrammatic view of the nature of a stress diagram for a wall of a vessel constructed in accord with this invention and forthe same working pressure as the Fig. 8 vessel;

Fig. 10 is a partial section of a pressure vessel of modied head construction;

Fig. 11 is a plan of apparatus for clamping the bands upon the inner shell prior to the welding of the opposite ends of the bands together;

Fig. 12 is an elevation of the apparatus shown in Fig. 11. showing the pressure vessel in transverse vertical section;

Fig. 13 is a sectional view of the hydraulic jack for the band applying device;

Figs. 14 and 15 are sections on the lines |4-|4 and ||5, respectively, of Fig. 13.

Fig. 16 is a transverse vertical section through a partially completed pressure vessel, illustrating also an embodiment in which each band is heated and then immediately welded while in heated condition;

Fig. 17 is a longitudinal vertical section on the line |1|1 of Fig. 18, showing one of the unit electrical heaters;

Fig. 18 is a plan of a heater unit;

Fig. 19 is a transverse section on line |9|9 of Fig. 18;

Fig. 20 is a detail View 'showing the manner in which each heating element is secured inthe heater;

Fig. 21 is a side elevation of the drum banding apparatus; and

Fig. 22 is an end elevation of the drum banding apparatus.

Referring to Fig. l of the drawings, the thickness of the inner shell is determined by the minimum thickness of the metal required for the hemispherical heads I0 and |2, these heads involving the most effective use of metal for a given working pressure.

As shown, the inner shell consists of a plurality of annular sections I4 and I6 made up of single plate sections formed to the diameter of the shell. For pressure vessels of large diameters each of these sections may involve a plurality of segments, but in the illustrative construction each section involves a single plate bent to circular form with its opposing ends united by a single longitudinal weld I0.

After the annular sections of the inner shell are fabricated, they are fixed in alignment and then united by a girth weld 20 to form the inner cylindrical shell. Thereafter the forged hemispherical heads 0 and |2 are similarly united with the inner shell by the circumferential welds 22 and 24.

All of the welds uniting the parts of the inner shell are then completely X-rayed as a check upon the quality of the welds. If defects are found the defective weld metal is removed and replaced. In this connection the present invention involves important advantages due to the fact that it permits the employment of metal plates much thinner than those which are required when the entire vessel is of a single plate construction. In the ilrst place, the quality of the thinner plates is apt to be much more uniform and otherwise superior to that of plates of greater thickness. In the second place, the X-rays of the welds, because of their relative thinness can be of high quality. In single plate constructions for very high pressures, the -thick plates arc joined by welds of such depth that the maximum effective range of the X-ray equipment is approached, if not exceeded. Thirdly, the cost of making X-rays of the thicker welds is consid-erably more than that for the smaller wclds, the time of making each X-ray varying directly with the square of the depth (or thickness) of the weld.

After it is determined that all of the welds of the inner shell are of the desired high quality, the shell is placed in a furnace and stress relieved. The shell is then allowed to cool before application thereto of the components of the outer shell.

As shown in the drawings. the outer shell consists of four series of bands 30, 32, 34, and 30. They are preferably thin plate sections which are separately formed `around the inner shell and tightly clamped thereon by the apparatus illustrated in Figs. 11-15. This apparatus includes a looped cable 40, the ends of which are anchored in clevises 42 and 44 while the middle part of the loop is seated within the groove 43 of the yoke or sheave 45 at the forward end of a base or frame 40. This frame is preferably of a heavy steel construction in order to successfully withstand the reactions of the mechanism carried thereby. That part of the frame extending from the hydraulic jack |00 to the left in Figs. 11 and 12 includes a body plate |02, the side web plates |04 and |00, the end plate |08, and the rear block ||0, all of which are preferably welded together in the relationships indicated.

At the top of the body plate |02 are eye rings ||2 and ||4 preferably welded to the body plate at the positions indicated. These rings are for the purpose of handling the entire band tightening mechanism and for moving it from one position to another along the drum upon which it is to be applied, and from one position to another in the shop. The eye ring ||2 is preferably located at the balancing point of the mechanism.

The clevis eye bolts 54 and 56 extend through openings in the rear block ||0 and are disposed on opposite sides of the body plate |02. They are held in their operative positions, and may be adjusted by the screw-threaded elements 50 and 52, threaded upon the end of the eye bolts and abutting against the rear surface of the heavy block ||0.

That part of the band tightener already described may be referred to as a backup strut, and at its forward end a heavy steel end plate |00 is maintained in a vertical position by welds which unite it with the forward end of the body plate |02 and the web plates |04 and |00.

That part of the band tightener frame at the right in Figs. 11 and 12 may be termeda bridge piece |23. It includes longitudinal side plates |20 and |22, rigidly bolted by the elementsl |30-I31 to the blocks |40 and |42, and similar elements rigidly welded to the end plate |00 and the body plate |02 of the backup strut. In order that the bridge piece may be readily and accurately placed in the correct position relative to the backup strut, assembly keys |50 and |52 are driven into openings each of which is partly formed by a channel in one of the side plates while its remainder is formed by one of the channels in the blocks |40 and |42, or other associated elements of the backup strut construction.

At the forward or right hand end (Figs. 1 1 and 12) of the bridge piece, there are upper and lower cross plates |54 and |56 preferably welded to the forward ends of the side plates and |22. As indicated in Figs. 13 and 15 of the drawings these cross plates carry xed guide blocks |60-I63. The guide blocks |60 and |6| are welded to the under side of the cross plate |54 and are spaced so that the guide member |66 'may slide in the space between them. A similar relationship exists between the lower guide member |68 and the guide blocks |62 and |63 which are secured to the upper side of the lower cross plate |56.

The parallel guide members |66 and |68 are rigidly secured to the yoke or sheave 45 in the positions illustrated in Fig. 13 of the drawings and this yoke is rigidly secured .to the forward end of the piston |10 of the hydraulic jack. The guide members |66 and |68 are preferably of heavy steel construction and are welded to the yoke 45 on its upper and lower surfaces, respectively. The guide member |66 has an upper projection |12 at its rearward end, and a similarly arranged downward projection |14 of the guide member |68 co-operates with the projection |12 and the cross plates |54 and |56 to limit the forward movement of the yoke 45 and thepiston |10 when Athe apparatus is being used to tighten a band upon the inner shell.

With the ends of the cable anchored within the clevises 42 and 44 and the intermediate portion of the cable wrapped around a band as indicated in Fig. l2 of the drawings, the band is tightened upon the inner shell by the flow of pressure fluid through the tube |80 and the duct |82 which communicates with the space |84 rearwardly of the piston |10 and within the hydraulic cylinder |86. Very high hydraulic pressures are involved in this operation, and leakage of the pressure fluid is prevented by the gland elements |90|91 secured at the forward or right hand of the cylinder |86, as indicated in Fig. 13.

A backing-up strip or band (Fig. 12) is interposed relative to the band 34 and the shell I6. It is not welded to the shell I6, and its opposite ends may be spaced from each other as indicated at 200 in Fig. 12. An important point with reference to the use of this backing-up strip is that itsends are remote from the position 202 at which the ends of the rst band 34 terminate.

Before the pressure of the band tightener is applied (with the elements in relative positions somewhat similar to those indicated in Fig. 12 of the drawings), the end 204 of the band 34 is positioned below the band tightener apparatus at such a distance that tack welding of the ends of the band 34 between the two parts of the band tightening cable, will be facilitated.

After the end 204 of the band 34 is positioned as shown, the band'tightener is lowered upon the band and adjusted to its desired' position by two sets of screw-threaded elements 2|0-2I4, there being a set on each of the side plates|04 and |06. i

As fluid enters the space |84 to force the piston |10 forwardly and tighten the cable, there is coordination of the cable stretching and the band bending actionto eliminate bends or folds in the band when it has received its final tightening upon the inner shell. The weight of the tightener has an `anchoring effect on the band` at the position RI` (Fig. l2) and the stretch of'the'cable is band 34 are made, a valve in an outlet tube communicating with the space |04 is opened. Thereupon, the cable is loosened upon'the drum and l the piston |10 is pushed back to the position in f which it is shown in Fig. 13 by the admission of fluid to the push back cylinders 220 and 222 formed by bores within the piston |10. This fluid passes to the cylinders 220 and 222 through the tubes 224 and 226 which are in communication with bores 228 and 230 formed in the rods 232 and 234. These rods are secured at their forward ends within openings in the guide blocks |6| and |63 in the manner indicated in Fig. 13, and their Jopposite ends are fitted within gland elements 24U-246 disposed between the rods and the metal of the piston |10 and secured in these positions by the cap screws 250 which are screw threaded into the piston.

The handling of the band tightening mechanism is also facilitated by the hook plates 252 and 254 (seeFig. 11) which are welded to the side plates |20|22 in the positions shown.

Successive bands such as the bands 3|-33 and 15-82 are secured in position in the manner above indicated, complete welds being formed between the adjacent ends of each tightened band immediately after the removal of the band tightener therefrom.

When the bands are considerably wider than the distance between the adjacent shell encircling head I2.

loops of the band tightener cable, the first band tightening and welding operations take place along the longitudinal center of a band and the successive band tightening and welding operations take place rst at one side of the zone of the first band tightening, and then at the other side.

' Preferably, the longitudinal welds I8 of the inner shell and the successive weldsf10-13 of the laminae of the outer shell are spaced circumferentiallyof theshell as indicated in Fig. 3 of the drawings. Thereare no circumferential welds at the junctions of the outer shell sections, as indicated in Fig; 4.

At one end of the pressure vessel, the ends of the outer shell laminae adjacent the head i2 preferably terminate at a plane including the line A-B which is a diameter of the hemispherical A similar relationship of elements applies at the other end of the pressure vessel, the line C-D (in the plane in which the adjacent ends ofthe adjacent laminae terminate) being a diameter of the hemispherical end ||J within a plane at which the adjacent bands terminate. With this construction, and with the effective number of bands applied adjacent the heads, all material of the pressure vessel is subject to the same unit stress, and no bending stress exists at the junctures of the heads and the cylindrical section of the shell.

Comparing the illustrative construction with a pressure vessel which is entirely constructed of such thin laminations as those 32, 34, and 36 of the present construction, and if it is assumed that the recognized construction code specifies a Wall thickness of 8" for the wholly laminated vessel. thenl for the same inside diameter, and

same pressure, the illustrative pressure vessel requires a wall thickness of only 6.4314 inches to maintain all stresses within code limitations. Such a comparison will be indicated from an inspection of Figs. 8 and 9 of the drawings. Fig. 8 illustrates the prior art construction where a laminated pressure vessel not stress-relieved and not X-rayed. for certain conditions of pressure and radius must have a wall thickness of approximately 8", the maximum permissible stress being 80% of 17,500 per sq. in., or 14,000 per sq. in. at the inner lamination, or at point R in Fig. 8.

Fig. 9 illustrates stress conditions in a wall of a vessel constructed according to this invention for the same radius and same working pressure. The Fig. 9 wall haga thickness of approximately 6.43" and maintains all stresses within the code limitations for the two types of welds employed. The maximum stress permissible on the inner shell which is stress-relieved and X-rayed, is 95% of 17,500 per sq. ini, or 16,625 per sq. in., and the maximum stress permissible at the point X, or at the first band (which is not stress-relieved and not X-rayed). is 80% of 17,500 per sq. in., or 14,000 per sq. in. It will be noted that the stress in the first band, or point X, is 13,859 per sq. in.,'somewhat within the value permitted by the code authorities.

Figs. 8 and 9 take into consideration the facts that the magnitude and variation of stress through the wall thickness of a cylinder under internal pressure follow certain natural laws, the stress being at a maximum on the inner shell surface, and at a minimum on the outer shell surface. The difference between the stress on the inner and outer shell surfaces is equivalent to the internal pressure. Further, the stress throughout the wall thickness varies as the square of the radius from the center of the cylinder.

Figs. and 6 indicate a construction to beeinployed when a nozzle section 63 is provided for a section of the drum. This nozzle section ex- 8 vessel with a drum head forged to provide a manway III. With this construction the wall of the head gradually increases in thickness from the end of the shell represented by the line TU, to the manway. The external surface of the head. to meet such conditions, may have a spherical surface of the radius R.' with its center in the line VW while the inner surface of the head may have a similar surface of the radius R. with a center in the middle of the line TU. In this modification, and for maintaining 'pmper conditions with reference to radial dilatation. the

bands of the outer shell 252 extend beyond welds tends through a side opening in the inner shell and is welded to a re-enforcing ring 6I, which in turn is welded to the shell, the elements 63 and i4 being preferably welded in'position before the inner shell is stress relieved.

Externally of the inner shell, and externally of the major laminations of the outer shell there are applied several additional bands 15 to 82, f

inclusive. These bands have openings for the nozzle section 63. The bands are clamped around the shell in the manner above described, being arranged so that the nozzle openings in successive bands will be substantially coaxial.

Each of the bands 'l5-82 is secured in the position in a manner similar to that above described. These bands are, as shown, of gradually decreasing width and their number is such that there is adequate compensation for the weakening of the drum metal occasioned by the provision of the nozzle opening.

After all of the additional bands 15-82 are applied the flanged nozzle section may be secured to the section 65 by the weld 66.

Fig. 7 also shows a drum provided with a manway 83 about which the head is reinforced by an internal ring 82 and an external ring 84. The former is constructed so as to provide a seat 86 for the manway cover 88 which is held in its drum closing position by means of through-bolts 90 and 92. These elements extend through a bar or plate 04 which contacts with the external plate structure I4.

at the ends of the line TU and to the piane of the line VW.

The use of a thicker inner-section and relatively thin re-enforcing bands reduces the tendency of the inner section to be strained through a difference in temperature of the contents of the vessel. Dividing the outer shell into separated sections also contributes to the minimizing of such strain due to a difference in expansion, and, combined with the thicker shell of the inner section, provides a. construction which is less subject to localstrains (which may cause trouble through corrosive action) than a vessel made up entirely of a number of relatively thin bands.

A modified method of fabricating such a vessel involves the preheating of each lamina before tightening. and the holding of the preheated band in tightened position upon the next innermost cylindrical surface during the welding of the longitudinal seam. When the entire circumferential length of each open lamina is so preheated. and the closure welding of the longitudinal seam to form a closed band is accomplished while the lamina is at an elevated temperature, the subsequentcontraction of the band tightly fitting an inner cylindrical surface results in a tensioning of the band, and the application of compression vforces to the contacted underlying cylinder.

The residual tensioning of the individual bands by temperature reduction from that occurring at the time when they are individually tightened and welded on a cylindrical surface, augments that which occurs in the previously described method of fabrication, where residual tensioning of bands results from the dilation of the inner shell upon release of the clamping and squeezing forces as applied by the cable, from the tension induced by the stretch of the tightening cables. and from the contraction upon cooling of the longitudinal extending fusion weld.

This manner of attaining an appreciable residual tensioning of the outer lamina is particularly advantageous when the design characteristics of the vessel indicate the selection of an inner shell of such a thickness that tightening forces of great magnitude and heavy apparatus would be required if the tensioning of the closed lamina were to result primarilyfrom the dilation of the inner shell after removal of the clamping and squeezing means.

With the use of apparatus which facilitates [introduction of the lamina preheating step, permitting the closing of the lamina to form the bands while under predetermined temperature conditions, it is possible to develop predetermined tension forces in the individual bands upon cooling, which are greater than the combined tension stresses otherwise effected.

' In the fabrication of the illustrative pressure vessel by the apparatus indicated in Figs. 16 to 22, inclusive, of the drawings. each band is temporarily secured about the shell 203 and it il thoroughly heated before it is tightened and welded. After heating, the band is tightened around the shell by a plurality of circumscribing ilat wire ropes, such as the rope indicated at 262 in Fig. 16. This rope is secured at one end to a fixed anchoring structure, indicated generally at 264, and its other end is secured through a shoe 266 and a shoe holder 268 to the piston 210 of an hydraulic cylinder 212.

Rope 262 has secured thereto at circumferentially spaced positions the unit heaters 214-211, the structure of which is indicated in Figs. 17-20. They are so disposed as to direct heat against a band, such as the band 280.

The band 280for example, is temporarily secured about the drum (and the underlying bands) with its ends slightly circumferentially spaced, at the position of the ultimate weld. as indicated at 282. This temporary positioning of the band may be attained by welding or such preforming of the band that it will remain in position around the shell, or the shell and its underlying bands, without any tightening oi' the rope 262.

After the rope 262 is in position, as shown in Fig. 16, the drum with the bands thereon is rotated past the fixed heaters so that the entire band 280 will be heated. After this is accomplished and the outer band has attained the desired temperature (for example. 150 FJ. the hydraulic mechanism 212 is operated to tighten the rope 262 on the band 280'and the opposed ends of the band are Welded together as at 282. This welding is done immediately, in order that there shall be-no substantial cooling of the band prior to welding.

The invention also comprehends theforming of each band in two semi-circular halves and the uniting of those halves by welding while they are held in shell circumscribing position.' In such case the weld holds the band in its position by reason of making of the weld in such a way that the underlying metal (shell or band) forms a part of the weld. In this event hydraulic tightening mechanism may be employed at each free end of each band so that the band would be pulled uniformly each way from its point of attachment to the underlying band, the stretch of the rope also being co-ordinated with the tightening of the band.

When the steel sheets for the bands are oi' a thickness of the order of 1A", or greater, each half band is preformed to closely t the underlying surface. For example, the band 280 is preformed in two semi-circular parts 280A and 280B which are then fitted upon the underlying band 294. While held in this position they are joined by the weld 280C which includes metal of the underlying band 294. This secures the band 280 to the shell through the underlying bands. After heating and tightening, the band 280 is completed by the weld at 282, the halves of the band being stiff enough to permit the use of the tightening apparatus shown in Fig. 16. In this mode of procedure it will be understood that the underlying bands 290-294 have been similarly installed, the preformed halves 290A and 290B of the first band, for example, being initially united and held on the shell 263 by the Weld 290C. After heating and tightening this band was completed by the weld 290D. The halves of the band 29| were likewise united by the welds 29IC and 29ID, in a similar procedure, and the remaining bands were installed in the same way.

With the above described procedure (involving the preheating of each band) the tension of each band 280 caused by the subsequent cooling of the weld, and that caused by the cooling of the band itself, are cumulative from astress standpoint, with respect to the stress imposed on the vessel by the tightening of the band by the hydraulic mechanism, the shell being put under compressive stress and the last installed bands under tension stress.

A plurality of the tensioning at wire ropes are utilized in the installation of each band. Six of such wire ropes are indicated in Fig. 21 of the drawings, four of them being disposed between the wire'ropes 262 and 262'. The cable or rope anchoring structure and the hydraulic mechanism are carried by a bridge 300 adjustably suspended from the top structure of a gantry 302 which is disposed so as to be movable along the rails 304 and 306. The bridge 300 is vertically adjustable on the gantry by means of manually controlled hoists 308 and 310, and in the illustrative procedure the bridge is elevated to permit the temporary installation of each band in its position, and thereafter. the bridge is dropped upon the band so that each one of the ilat wire ropes (such as 262 and 262'), may be secured in its operative position bv bringing the opening in the rone anchor clevis 312 into register with the coacting opening in the clevis 3|4. Thereupon the pin 3l6 is threaded through these openings to hold the rope and its heaters in their operative positions, it beiner understood that each ol' the several ropes is similarlv `secured in operative position along with its heaters.

To provide for the rotation of the drum during the uniform heating of a band one end of the drum is secured to a head 320, which is mm1-amv mounted in a bearing at the ton of a pedestal 322.

At its other end the drum is fixed to a drum adaptor. indicated generally at 324. This is of heavv plate structure with a main vertical plateV 326 to which are welded axially extending plates, such as 328 and 330. The latter may be convenientlv attached to drum by welding as indieafed. Fixed to thenther side oi' the main nlate 326 is a heavv annular structure 332 having an unstanding ring 334 supported bv the rotatably mounted flanged rollers 336 and 338. These rollers are driven by a motor 340, with speed reducing mechanism 342 interposed relative to the motor and the rollers.

One end of each flat wire rope, such as rope 262. is anchoredby a structure, such as that indicated. in Fig. 16. This inf-.indes a cable clamp base 350 with a clamp top 352, the latter being tightly bolted to the base 350 and the body of the structure 264. Beyond the clamp base 350 the rope is anchored to a pin 354 which passes through the eye of an anchor bolt 356, secured at its other end to a plate 358 which is rigid with the structure 264.

vThe heaters carried by each of wire ropes, such as 262 and 262', are of the electrical resistance type, their structure being indicated in Figs. 17 to 20. Each includes a heat resistant body 400 with parallel grooves in its heating space for the reception of the rod-like heatingv elements 402-401, each of these elements being disposed with reference to the body 400 in the manner indicated in Figs. 17-20. The upwardly bent ends of the rods, such as 408 and 4|0,l are disposed within housings 4I2 and 4I4 in which the electrical connections are made. These lconnections are such that the heating rods are connected in parallel. Each rod is held in its groove in me manner indicated in m. 2o. 'rms indrical conilguration. Between the heaters, the

ropes make contact with the outermost band and direct pressure directly upon the underlying structure.

The refractory body 49| is embraced by a metal backing 430 with insulation material 433 interposed, as indicated at Fig. 19.

The above indicated method of manufacture is particularly adapted for the construction of heavy pressure vessels in which the thickness of the inner shell may run as high as 6 inches or higher. In such a vessel the total thickness of the wall including the inner shell and the bands may be greater than 12 inches, and the individual bands may be of such thickness as one-quarter inch.

'I'he welds of adjacent bands are preferably offset for reasons referred to in the early part of the specification, and other characteristics and attributes of the first described procedure and the resulting pressure vessel also apply to the procedure and the pressure vessel last described.

summarizing the fabrication method above described, it involves:

(a) the formation of a welded shell, or pressure vessel, of relatively thick walls,

(b) stress relieving the shell or pressure vessel,

(c) preforming relatively thin plate or sheet sections to curvature of the order of the shell curvature,

(d) successively fitting the sheet sections upon the shell and (and any underlying shell and band structure) securing them thereon,

(e) uniformlyheating each sheet section upon the shell by rotating the shell and the band through one or more heating zones,

(f) tightening each heated sheet section upon the shell by circumscribing tensioning forces of Aconsiderable magnitude,

'The specific apparatus for the manufacture of multi-layer pressure vessels, including particularly the band tightening means is not claimed in this application, but isclaimed in my copending application Ser. No. 95,363, filed on May 25. 1949.

What is claimed is:

A method of manufacturing multi-layer pressure vessels, the method comprising theforming of an inner cylindrical steel shell, positioning an incomplete and untightened circumscribing steel plate band over and around the shell with the ends of the band spaced from each other circumferentially of the shell, heating said positioned incomplete band to a predetermined degree throughout its area before the band is completed by joining its ends, said heating being accomplished by establishing relative rotation between the band and spaced heating zones, tightening the heated and untightened incomplete band around the shell before any substantially cooling of the band occurs, said heating and tightening of the band reducing the circumferential spacing of the band ends to a valve within seam welding range, welding together the band ends along the line of the longitudinal weld seam while the heated band is held tightened around the shell, cooling the band and thereby increasing its tension and its compressive effect upon the shell, and repeating the above specified band procedure in superposing successive bands over and around the first band.

CECIL H. GAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS Number Name Date 1,301,995 Arthur Apr. 29, 1919 1,365,198 Sessions Jan. 11, 1921 1,529,685 Beugler Mar. 17, 1925 1,663,255 Hynes Mar. 20, 1928 1,704,413 Wait Mar. 5, 1929 1,770,763 Wolfstyn July 15, 1930 1,920,791 Hogan Aug. 1, 1933 1,947,462 Doorbar Feb. 20, 1934 1,975,422 Hellenbroich Oct. 2, 1934 2,217,090 Zerbe Oct. 8, 1940 2,219,085 Watson Oct. 22, 1940 2,348,696 Schabacker May 9, 1944 2,359,446 Scudder Oct. 3, 1944 2,372,723 Jasper Apr. 3, 1945 2,376,351 Gay May 22, 1945 FOREIGN PATENTS Number Country Date 342,945 Great Britain Feb. 12, 1931 37,761 Denmark July 19, 1927 

